Green Card for Parent in US - they want Supplement A to Form I-485

[Ion Tichy]

I am sponsoring my mother to get Green Card.

All forms have been filled and she just completed biometrics.

Now, we received a letter that they want Supplement A to Form I-485 and $1000 penalty fee.

Why? Do we have to do that form and pay $1000. Mom is currently on legal/valid visa in US with us.

Please advise

Thanks